Electromagnetic actuators, such as linear or rotary solenoids, typically include a coil in electromagnetic communication with a movable armature. The coil is generally connected to a controllable driving circuit which varies the magnitude of current flowing in the coil and resultantly varies the strength of the magnetic field being produced by the coil. As the strength of the magnetic field is changed, the armature moves in response to the resulting change in the magnetic force being exerted on the armature.
Typically, the position of the armature is a function of both the magnitude of current flowing in the coil and the magnitude and direction of mechanical forces being exerted on the armature. The mechanical forces are exerted on the armature in response to the operating conditions of the system in which the electromagnetic actuator is operating. It is therefore advantageous to have a method of determining the position of the armature so that the operating conditions of the system can be indicated and used in connection with system diagnostics or a closed-loop control for the driving circuit.
The most common method of determining the position of the armature of an electromagnetic actuator is to connect an external sensor to the actuator. Such sensors often take the form of potentiometers or linear voltage differential transformers (LVDTs). While the addition of these sensors provides the desired information, they increase the cost and warehousing requirements of the actuator.
Attempts to provide position information without utilizing additional sensors have generally taken the form described in Japanese Patent Appl. No. 61-157418, published Jan. 20, 1988, and in Proceedings: 39th Relay Conference, Apr. 22-24, 1991, National Association of Relay Manufacturers, pp. 9-1 through 9-4. Both of the above publications disclose systems which determine the position of the armature by measuring the inductance of the coil in the actuator. Since inductance is a function of the air gap between the armature and the coil, the size of the air gap, and hence armature position, is determined by comparing measured inductance values to empirically determined inductance versus position characteristics. Systems of this type are generally incapable of providing accurate results for actuators exhibiting second-order characteristics since such actuators do not have explicit inductance values. Furthermore, as described in the Japanese Application, additional measurements and comparisons, e.g. coil temperature and magnetomotive force, are required to provide accurate indications of armature position.
Devices for determining armature position are particularly useful in connection with solenoids and electrohydraulic valves. Electrohydraulic valves are often used to control the engagement/disengagement of transmission clutches. The engagement of a hydraulic clutch consists of two stages: the fill mode and the pressure modulation mode. In the fill mode, the displaced clutch volume is filled with hydraulic fluid. In the pressure modulation mode, the pressure within the clutch volume is modulated (increased) to a pressure level to ensure proper and full engagement of the clutch. To actuate the clutch, the solenoid is therefore, first energized to begin filling the clutch. When the clutch is filled, the current applied to the solenoid is modulated (typically, in an increasing linear ramp function) to continue the flow of hydraulic fluid to the clutch and increase the pressure to a level sufficient to properly engage the clutch.
Typically, a timing strategy has been used to determine when the clutch has reached the end of fill condition. In this situation, the solenoid's coil would be energized and the clutch would begin to fill with hydraulic fluid. After a predetermined time period, the transmission controller would begin to modulate current, in an effort to fully engage the clutch.
This procedure has several limitations. For example, operating conditions change the actual time required to fill the clutch. Since pump flow is a function of engine speed, pump flow will vary with engine speed. Other factors (for example, other hydraulic systems being supplied by the pump) may also affect pump flow. As the pump flow varies, the time required to fill the clutch will also vary. Other operating conditions which affect the clutch fill times are present gear ratio, desired gear ratio, transmission load, and inclination of the vehicle.
Variations in the engine and operating characteristics of the transmission components can be expected over the life of the vehicle due to wear. This will also affect the clutch fill time.
Furthermore, variations in the system components, including the clutches, due to manufacturing tolerances will also affect clutch fill time.
If the proper fill time is not known or accurately estimated, the clutch will be in an overfill or underfill condition when the controller attempts to modulate clutch pressure to fully engage the clutch.
Operation of the transmission by modulating the clutch pressure in an underfill or overfill condition will cause a "jerky" shift action and increase the rate at which wear and tear occurs.
In an attempt to predict fill times, sensors are often added to the transmission controller. For example, U.S. Pat. No. 4,707,789 issued to Robert C. Downs et al., on Nov. 17, 1987, uses a transmission input speed sensor to detect underfill/overfill condition. The time delay used to estimate clutch fill is adjusted based upon the transmission input speed. However, transient changes, that is, changes in the operating conditions that the controller has not adapted to, will affect the shift quality. Furthermore, a transient condition will have a negative effect on the fill time for the next shift without the transient condition.
In another attempt to accurately predict the end of fill condition, it is known to add additional valves to the controller. One such system is shown in the Komatsu technical guide, "K-ATOMICS Komatsu-Advanced Transmission with Optimum Modulation Control". A flow sensing valve is used to sense a pressure differential. The spool of the flow sensing valve closes a switch in response to the pressure differential, thereby, signalling the end of fill condition. In still another attempt, hydraulic pressure is used to predict the end of fill condition. U.S. Pat. No. 4,942,787 issued to Takashi Aoki et al., on Jul. 24, 1990, discloses the use of a pressure detection switch for that purpose. However, the cost added by the additional components in both these systems, plus, the added manufacturing cost due to the increased complexity, make these systems undesirable.
The present invention is directed at overcoming one or more of the problems as set forth above.